The invention is potentially applicable to any such craft, for example a fixed wing propeller-driven aircraft, a propeller-driven submarine or water surface craft, However, the invention arose in connection with the design of a particular type of vertical take-off aircraft where a bladed rotor is used to direct a jet of air over a curved surface so that the jet is turned from a radial direction towards an axial direction thereby creating the required lift or thrust.
Stability is a particularly acute problem in craft such as helicopters which are required to hover. Complex control systems are normally needed to achieve the required degree of stability. For craft of the particular type referred to above, no satisfactory stabilising system has yet been devised and, probably for that reason, such craft have at least mainly been confined to paper proposals and unsuccessful experimental models.